1. Field of the Invention
The present invention relates to the use of a nanocrystalline layer of Cu2O in the construction of photovoltaic cells to increase the ability of the photovoltaic cells to utilize UV radiations for photocurrent generation.
2. Discussion of the Art
The solar cell is considered a major candidate for obtaining energy from sun, since it can convert sunlight directly to electricity with high conversion efficiency, can provide nearly permanent power at low operating cost without having any influence on the climate. Recently, research and development of alternative energy technologies, specially, low cost, flat-panel solar cells, thin film devices, concentrator systems, and many innovative concepts have increased.
In a solar cell, the bandgap determines the conversion efficiency and the region of the solar spectrum (that is close to a black-body spectrum at temperature T=5800 K) it covers. According to the air-mass-zero (AMO) (assuming no absorption of radiation) spectra, the intensity is maximum around 400 to 600 nm. The primary requirement for a material to be applicable to solar cells is a bandgap matching the solar spectrum and high mobilities and lifetimes of charge carriers. Solar cells made of inorganic materials like Si, GaAs, CdTe, InP, etc., covers only a small fraction of the solar spectrum in the visible region (for example, the cut-off wavelength for GaAs and Si being 0.87 μm and 1.1 μm respectively). In a solar cell, for conversion efficiency, it is important to note that photons which have energy ℏω smaller than the semiconductor bandgap will not produce any electron—hole pairs. Also, photons with energy greater than the bandgap (Eg) will produce electrons and holes with same energy (Eg) regardless of how large (ℏω−Eg) is. The excess energy ℏω−Eg is simply dissipated as heat. Thus the solar cell efficiency depends quite critically on how the semiconductor bandgap matches with the solar energy spectra.
In an effort to increase the conversion efficiency of photovoltaic devices, thermo-photovoltaic (TPV) devices were developed, which converts thermal radiations (infra-red radiations) to electricity. In this way, a substantial part of the solar spectrum was utilized for increasing the conversion efficiency and operating wavelength range. These devices are mostly heterojunction devices consisting of both wide and low bandgap materials. The energy gaps in these cases were varied from 0.3 to 0.7 eV, thereby covering long wavelength regions of the solar spectrum.
Major disadvantages with the TPV devices are that they are heterojunction devices requiring expensive epitaxial growth techniques for its fabrication. The necessity for epitaxial growth techniques arises from the fact that the materials require lattice matching between different materials involved in the structure.
The intensity of the solar spectrum is maximum in the Ultra-violet region. Presently, due to unavailability of proper lattice matched substrates for wide band gap semiconductors like GaN (Gallium Nitride), solar cells operating in the UV region of the solar spectrum is yet to be developed. The relation between the open-circuit voltage and the efficiency of a cell is given by the equationη=(IscVocFF/Iincident)  (1)where, Isc is the short circuit current, Voc is the open-circuit voltage and FF is the fill factor. Iincident is the intensity of the incident radiation. For AMO, Iincident is 140 mW/cm2 As can be seen from the equation, increase in intensity of the solar radiation increases the short circuit current of the cell. This in turn will lead to an increase in the efficiency of the cell. Use of lattice mismatched substrates results in existence of large defect densites, which in turn reduces the open-circuit voltage, and hence the efficiencies of the cell. It is quite well known that open-circuit voltage is limited by extrinsic recombination processes such as through bulk defect levels, through surface defect levels, and at metallic contacts to the cell.
When a monochromatic light of wavelength λ is absorbed in a solar cell, the photon flux in emitter, depletion region and base region will generate electron hole pairs which are accelerated by the junction electric field and collected in the front and back metal grids. Generation rate G(λ,x) of these carriers at a distance x from the front emitter surface is given by,G(λ,x)=αλNph(λ)e−∝  (2)where αλ is the absorption coefficient of the incident light in the solar cell material, Nph is the number of photon per unit area per second. Spectral response of a solar cell is given by the probability that the absorbed photon will yield a carrier for the photogenerated current of the cell. It might be good to account the feature that photocurrent, Jsc, of the cell for a given irradiance level E(λ) with its spectral distribution E(λ) dλ is given by,Jsc=qNph∫[I−Rλ]SR(λ)dλ  (3)where Rλ is the front surface reflectivity and SR is the spectral response of the cell. This indicates that the increase in intensity of the solar spectral radiation and the SR increases the short circuit current density (Jsc) of the cell. Hence, the key to improving the efficiency of the cell lies in increasing both the short-circuit current and open-circuit voltage of the cell.
Efficient dye-sensitized photovoltaic devices employing nanocrystalline metal oxide films were first reported in 1991, see B. O'Regan and M. Gratzel, Nature, 1991, 353, 737. Such cells can achieve solar to electrical energy conversion efficiencies of up to 10%. There is currently extensive commercial R&D aiming to develop photovoltaic devices based upon this design, centered around Prof. Gratzel's group in EPFL, Switzerland. Previous investigations have searched for the identity of the optimum sensitizer dye for this technology.
An important limitation in the design of dye-sensitized PV devices currently being commercialized is the requirement for an electrically conducting liquid component (an electrolyte). It has been proposed to replace this liquid with a solid-state eletrolyte analogue. The important requirements for such solid-state analogues are good device energy conversion efficiency, good stability and low sealing requirements. Gratzel and co-workers, have reporting that an organic material, OMeTAD developed for the Xerographic industry, is one such material, although the efficiency of the solid state device produced with this material was limited to <0.8%, see Back, et al., Nature 1998, 395, 583. Other proposals have considered conducting polymers but have to date achieved efficiencies even less than the above, see Murakosh, et al., Chem Letts., 1997, 471. Polymer gel electrolytes have also been shown to achieve high efficiencies, however such systems retain a solvent phase and therefore still require sealing, and cannot therefore be regarded as truly sold state. See Cao, et al., J. Phys. Chem, 1995, 99, 18071. There has been a report of the use of solid-state ionic commercial rubber, but efficiencies achieved were very low (0.1%), see Nogueira, et al., Abstracts IPS-12.
Various photovoltaic and battery cells are described in U.S. Pat. No. 5,441,827, U.S. Pat. No. 5,438,556, U.S. Pat. No. 4,520,086, International Publication No. WO 97/08719, and International Publication No. WO 93/20569.
Dye-sensitized solar cells are more and more maturing into a technically and economically credible alternative to the conventional p-n junction photovoltaics. Photoelectrochemical (PEC) liquid junction cells based on the photosensitization of semiconductor TiO2 layers with molecular sensitizers attracted renewed interest after Gratzel, et al. reported energy conversion efficiencies >10%, in Nature, 353 (1991) 737. In 1998, Gratzel, et al. reported in Nature, 395 (1998) 583, a more innovative all solid-state cell based on a heterojunction sensitized by a molecular dye where an amorphous organic whole transport material replaced the liquid electrolyte.
The crucial part in these cells is the dye itself. Only a very limited number of dyes give high photocurrent quantum yields and are reasonably stable against photo-degradation. Some of the organic dyes exhibiting high light absorption are sensitive to air and water.
It is a well known phenomena that a semiconductor nanostructure exhibits a strong blue-shift (increase in the band gap energy) in the band gap due to quantum confinement effects. The band gap distribution is obtained by assuming a distribution of sizes for d (diameter of the nanostructures) and a relation governing the upshift in energy ΔE with the size d (due to quantum confinement). Considering, two possible distribution of sizes for the nanostructures, Gaussian and lognormal, given as             P      G        ⁡          (      d      )        =            1              √                  (                      2            ⁢                                                   ⁢            π            ⁢                                                   ⁢            σ                    )                      ⁢                   ⁢          Exp      ⁡              (                  -                                                    (                                  d                  -                                      d                    O                                                  )                            2                                      2              ⁢                                                           ⁢                              σ                2                                                    )            where dO is the mean size and the σ is the standard deviation, and             P      L        ⁡          (      d      )        =            1                        σ          L                ⁢        d        ⁢                  √                      (                          2              ⁢                                                           ⁢              π                        )                                ⁢                   ⁢          Exp      ⁡              [                  -                                                    (                                                      ln                    ⁡                                          (                      d                      )                                                        -                                      m                    O                                                  )                            2                                      2              ⁢                                                           ⁢                              σ                L                2                                                    ]            where mO=ln(dO), and σL=ln(σ), respectively.
Thus the energy shift for the confinement ΔE can be written asΔE=EG−Eg=C/dγ  (6)where, Eg is the crystalline fundamental band gap of the nanocrystals and EG is the increased band gap due to quantum confinement. The distribution of the band gaps of nanostructures P(EG) can be obtained by making a change of variable from d to EG in the distribution functions given in equations (4) and (5). This effect of energy shift to higher energy due to size quantization, with carriers being confined to essentially “zero” dimensional structure makes nanostructures a promising candidate for opto-electronic devices.
All nanostructures materials share three fundamental features of (i) atomic domains (crystalline core) spatially confined over 2 to 20 nm, (ii) significant atom fractions associated with interfacial environments, and (iii) interactions between their constituent domains. In so small particles, 60-20% of the total atoms form the surface of the particles (intercrystalline region). The fraction of the surface atoms (Φ) is represented as a potential well with finite potential height (VO) and the barrier representing the intercrystalline region. For finite barrier quantum structure, the binding energy (Eb) first increases when the cluster size is reduced. It reaches a maximum for a critical size of a cluster of a given material. Then the binding energy starts reducing once again even if the cluster size is reduced further. FIG. 1 shows a general configuration for a nanostructure.
Commercially available techniques for growth of quantum structures, like Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD) are very expensive and require special handling capability. The modern inorganic photovoltaic technology uses some highly toxic materials such arsenic and phosphine compounds and specialized growth techniques for fabrication. Currently, most of the research focuses on low-cost photovoltaic devices using expensive thin film technology with TiO2 as the optical window coating for fabrication. But the efficiency of these cells is yet to cross the 10% threshold limit for domestic applications.
Mitra, et al. (Mat. Sci and Eng., A304-306 (2001) 805-809) has described the development of Cu2O nanostructures by Cu2+→Cu→Cu+ redox reaction in an aqueous medium by adding an aqueous NaBH4 solution (a reducing agent) to an aqueous Cuprous Chloride (CuCl2) solution at 80-100° C. As reported by Mitra, et al., the x-ray diffraction pattern represents an orthorhombic structure, which is significantly different from that of the well-known FCC structure of bulk Cu2O. The ellipsoidal shape of Cu2O granules in the TEM micrograph shown in FIG. 2A is demonstrative of a modified morphology. The Cu2O nanocrystals exhibit a strong blue shift of the optical bandgap around 4.8 eV with respect to 2.1 eV in the bulk in the electronic absorption spectrum by a strong quantum confinement of the electronic charges. Using quantum structures, the entire UV part of the solar spectrum is used for improving the “short-circuit current” of the photovoltaic cell.
It is therefore a target of the present invention to propose photovoltaic cells having improved stability against photo-degradation and environmental influences. It is another target of the invention to achieve higher photocurrent quantum yields, higher photovoltages and hence higher conversion yields using the UV part of the solar spectrum.